1. Field of the Invention
The present invention relates to an encoder output signal correction apparatus and method for correcting two-phase sinusoidal signals output from an encoder that detects positions, angles, velocities, angular velocities and so forth.
2. Description of the Related Art
An interval between gratings formed on a scale in an encoder is limited to process. In order to measure a finer interval than that between the scale gratings, the spatial period of a phase variation in a sinusoidal signal output from the encoder should be divided much finer and interpolated. A variety of interpolators have been employed in the art. For example, a digitized interpolator comprises an A/D converter and a memory. The A/D converter is operative to receive A-, B-phase sinusoidal signals with a phase difference of 90° output from the encoder and sample the signals at a certain frequency to convert them into digital data. The memory is employed to store a look-up table for use in searching phase angle data PH at each sampling point based on the digital data DA, DB obtained at the A/D converter. The look-up table is created based on PH=ATAN (DA/DB) using a function of arctangent (ATAN).
The A-, B-phase sinusoidal signals output from the encoder are usually not complete sinusoidal waves and can be represented in an orthogonal coordinate system as describing an elliptical Lissajous waveform generally. When the A-, B-phase sinusoidal signals have different amplitudes in voltage, the Lissajous waveform becomes elliptical. An offset value of each signal voltage turns the Lissajous waveform into a circular or elliptical waveform deviated from the origin. The presence of a phase error brings the major and minor axes of an ellipse into a state not parallel with the coordinate axes but close to 45°. An interpolator is made on the assumption that the A-, B-phase sinusoidal signals are sinusoidal waves. Accordingly, a deviation from an ideal sinusoidal wave exerts an ill effect on the interpolation accuracy. In order to correct amplitude errors, phase errors and offsets in the A-, B-phase sinusoidal signals, devices for the purpose have been proposed in JP-A 10-311741 and JP-A 2003-222534, for example.
In the two-phase sinusoidal signals after such the amplitude error and so fourth are corrected, however, a deviation from an ideal sinusoidal signal waveform, or a waveform distortion, is large, and the distortion factor fluctuates greatly in particular as the interval between a main scale and an index scale fluctuates. The waveform distortion is mostly caused by higher harmonic components of odd degrees (third, fifth degree, and so forth). When the two-phase sinusoidal signals with such the fluctuating distortion factor is employed in measurements, a large measurement error arises.
Several technologies have been proposed to provide sinusoidal signals from which such the higher harmonic components are removed. For example, JP-A 3-48122 proposes a technology, which provides two rectangular grating patterns with a slight phase difference on a scale and sums the outputs therefrom to just cancel higher harmonic components. JP Patent No. 2695623 proposes another technology, in which a scale with uniform gratings and a scale with non-uniform gratings are combined to provide sinusoidal signals from which such the higher harmonic components are removed.
The technology of JP-A 3-48122 reduces the waveform distortion to some extent though the accuracy is not sufficient for application to position measurements in precise machining or the like. The system of JP Patent No. 2695623 requires precise creation of a duty factor between light and dark portions while the precise creation is difficult in a fine scale.
The present invention has an object to provide an encoder output signal correction apparatus and method capable of removing a third harmonic component contained in a sinusoidal signal using relatively simple digital computations, thereby improving the interpolation accuracy in an interpolator. It is also possible to improve robustness against offset errors, amplitude errors, phase errors and higher harmonic component errors due to non-uniformity of scales and non-uniformity of alignments in an encoder.